1. Technical Field
Therapeutics are needed for treating diseases and disorders related to aberrant cystic fibrosis transmembrane conductance regulator protein (CFTR)-mediated ion transport, such as polycystic kidney disease, increased intestinal fluid secretion, and secretory diarrhea. Small molecule compounds are described herein that are potent inhibitors of CFTR activity and may be used for treating such diseases and disorders.
2. Description of the Related Art
The cystic fibrosis transmembrane conductance regulator protein (CFTR) is a cAMP-activated chloride (Cl−) channel expressed in epithelial cells in mammalian airways, intestine, pancreas, and testis (see, e.g., Sheppard et al., Physiol. Rev. 79:S23-45 (1999); Gadsby et al., Nature 40:477-83 (2006)). Hormones, such as a β-adrenergic agonist, or a toxin, such as cholera toxin, lead to an increase in cAMP, activation of cAMP-dependent protein kinase, and phosphorylation of the CFTR Cl− channel, which causes the channel to open. An increase in cell Ca2+ can also activate different apical membrane channels. Phosphorylation by protein kinase C can either open or shut Cl− channels in the apical membrane. CFTR is predominantly located in epithelia where it provides a pathway for the movement of Cl− ions across the apical membrane and a key point at which to regulate the rate of transepithelial salt and water transport.
CFTR chloride channel function is associated with a wide spectrum of disease, including cystic fibrosis (CF) and with polycystic kidney disease, secretory diarrhea, and some forms of male infertility. Cystic fibrosis is a hereditary lethal disease caused by mutations in CFTR (see, e.g., Quinton, Physiol. Rev. 79:S3-S22 (1999); Boucher, Eur. Respir. J. 23:146-58 (2004)). Observations in human patients with CF and mouse models of CF indicate the functional importance of CFTR in intestinal and pancreatic fluid transport, as well as in male fertility (see, e.g., Grubb et al., Physiol. Rev. 79:S193-S214 (1999); Wong, Mol. Hum. Reprod. 4:107-110 (1997)). CFTR is also expressed in enterocytes in the intestine and in cyst epithelium in polycystic kidney disease (see, e.g., O'Sullivan et al., Am. J. Kidney Dis. 32:976-983 (1998); Sullivan et al., Physiol. Rev. 78:1165-91 (1998); Strong et al., J. Clin. Invest. 93:347-54 (1994); Mall et al., Gastroenterology 126:32-41 (2004); Hanaoka et al., Am. J. Physiol. 270:C389-C399 (1996); Kunzelmann et al., Physiol. Rev. 82:245-289 (2002); Davidow et al., Kidney Int. 50:208-18 (1996); Li et al., Kidney Int. 66:1926-38 (2004); Al-Awqati, J. Clin. Invest. 110:1599-1601 (2002); Thiagarajah et al., Curr. Opin. Pharmacol. 3:594-99 (2003)).
Polycystic kidney disease (PKD) is one of the most common human genetic diseases and a major cause of chronic renal insufficiency requiring dialysis and kidney transplantation (see, e.g., Torres et al., Lancet. 369, 1287-301 (2007)). Cyst growth in PKD involves fluid secretion into the cyst lumen coupled with epithelial cell hyperplasia. PKD is characterized by massive enlargement of fluid-filled cysts of renal tubular origin that compromise normal renal parenchyma and cause renal failure (see, e.g., Arnaout, Annu. Rev. Med. 52: 93-123, 2001; Gabow N. Engl. J. Med. 329: 332-342, 1993; Harris et al., Mol. Genet. Metab. 81: 75-85, 2004; Wilson N. Engl. J. Med. 350: 151-164, 2004; Sweeney et al., Cell Tissue Res. 326: 671-685, 2006; Chapman J. Am. Soc. Nephrol. 18: 1399-1407, 2007). Human autosomal dominant PKD (ADPKD) is caused by mutations in one of two genes, PKD1 and PKD2, encoding the interacting proteins polycystin-1 and polycystin-2, respectively (see, e.g., Wilson, supra; Qian et al., Cell 87: 979-987, 1996; Wu et al., Cell 93:177-88, 1998; Watnick et al., Torres et al., Nat Med 10: 363-364, 2004 Nat. Genet. 25: 143-44 (2000)).
Cyst growth in autosomal dominant polycystic kidney disease (ADPKD) involves progressive fluid accumulation (see, e.g., Grantham et al., Clin. J. Am. Soc. Nephrol. 1:148-57 (2006); Ye et al., N. Engl. J. Med 329:310-13 (1993)). Fluid secretion into the cyst lumen requires chloride secretion by the cystic fibrosis transmembrane conductance regulator (CFTR) protein, (see, e.g., Hanaoka et al., J. Am. Soc. Nephrol. 11:1179-87 (2000); Magenheimer et al., J. Am. Soc. Nephrol. 17:3424-37 (2006)), a cAMP-regulated chloride channel, which, when mutated, causes the genetic disease cystic fibrosis (see, e.g., Riordan, Annu. Rev. Biochem. 77:701-26 (2008)). CFTR is expressed strongly in epithelial cells lining cysts in ADPKD (see, e.g., Brill et al., Proc. Natl. Acad. Sci. USA 93:10206-11 (1996)). Cystic fibrosis (i.e., CFTR-deficient) mice are resistant to cyst formation and CFTR inhibitors block cyst formation in cell/organ culture and in vivo models (see, e.g., Davidow et al., Kidney Int. 50:208-18 (1996); Li et al., Kidney Int. 66:1926-38 (2004)). In rare families affected with ADPKD and cystic fibrosis, individuals with both ADPKD and CF have less severe renal disease than those with ADPKD only (see, e.g., Cotton et al., Am. J. Kidney Dis. 32:1081-83 (1998); O'Sullivan et al., Am. J. Kidney Dis. 32:976-83 (1998); Xu et al., J. Nephrol. 19:529-34 (2006)).
Several CFTR inhibitors have been discovered, although many exhibit weak potency and lack CFTR specificity. The oral hypoglycemic agent glibenclamide inhibits CFTR Cl− conductance from the intracellular side by an open channel blocking mechanism (see, e.g., Sheppard et al., J. Physiol., 503:333-346 (1997); Zhou et al., J. Gen. Physiol. 120:647-62 (2002)) at high micromolar concentrations where it affects other Cl− and cation channels (see, e.g., Edwards & Weston, 1993; Rabe et al., Br. J. Pharmacol. 110:1280-81 (1995); Schultz et al., Physiol. Rev. 79:S109-S144 (1999)). Other non-selective anion transport inhibitors, including diphenylamine-2-carboxylate (DPC), 5-nitro-2(3-phenylpropyl-amino)benzoate (NPPB), and flufenamic acid, also inhibit CFTR by occluding the pore at an intracellular site (see, e.g., Dawson et al., Physiol. Rev., 79:S47-S75 (1999); McCarty, J. Exp. Biol., 203:1947-62 (2000)).
High-affinity CFTR inhibitors also have clinical application in the therapy of secretory diarrheas. Secretory diarrheas caused by enterotoxins, such as cholera and Travelers' diarrhea (enteropathogenic E. coli), require functional CFTR for primary chloride secretion into the intestinal lumen, which secondarily drives sodium and water secretion (see, e.g., Kunzelmann et al., Physiol. Rev. 82:245-89 (2002); Thiagarajah et al., Curr. Opin. Pharmacol. 3:594-9 (2003)). Cell culture and animal models indicated that intestinal chloride secretion in enterotoxin-mediated secretory diarrheas occurs mainly through CFTR (see, e.g., Clarke et al., Science 257:1125-28 (1992); Gabriel et al., Science 266:107-109 (1994); Kunzelmann and Mall, Physiol. Rev. 82:245-89 (2002); Field, J. Clin. Invest. 111:931-43 (2003); and Thiagarajah et al., Gastroenterology 126:511-519 (2003)). Several classes of small molecule CFTR inhibitors have been described previously (see, e.g., review by Verkman et al., Nat. Rev. Drug Discov. 8:153-71 (2009)).
Diarrheal disease in children is a global health concern: Approximately four billion cases among children occur annually, resulting in at least two million deaths. Travelers' diarrhea affects approximately 6 million people per year. Antibiotics are routinely used to treat diarrhea; however, the antibiotics are ineffective for treating many pathogens, and the use of these drugs contributes to development of antibiotic resistance in other pathogens. Oral replacement of fluid loss is also routinely used to treat diarrhea, but is primarily palliative. Therapy directed at reducing intestinal fluid secretion (‘anti-secretory therapy’) has the potential to overcome limitations of existing therapies.
A need exists for CFTR inhibitors, particularly those that are safe, non-absorbable, highly potent, inexpensive, and chemically stable.